Imaging devices, such as digital cameras and surveillance TV cameras, conventionally use image detecting elements such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) image detecting device. In recent years, these image detecting devices have evolved so that they provide a high resolution detector in a compact configuration. Such performance demands that the imaging lens itself, which is used with such detectors be both compact and have a large aperture ratio or F-number so as to provide a bright image. Additionally, imaging lenses that provide wide angles of view are demanded. Furthermore, in some imaging devices, a so-called low-pass filter is often placed between the imaging lens and the image plane in order to reduce the high spatial frequency components of the object light for imaging. Therefore, an appropriately long back focus is needed in order to provide room for such a low-pass filter.
In order to satisfy the demands noted above, a retrofocus imaging lens which has a large aperture ratio and a wide angle of view, as well as a fixed focal length, may be designed. Known imaging lenses designed to meet the demands noted above are described in Japanese Laid-Open Patent Applications H09-166748, H11-142730, 2000-047101, and 2000-105334. The fixed focal length lenses described in these applications all consist of five lens elements, with the third and fourth lens elements from the object side forming one of four lens components.
However, the lenses described in these applications are not adequate in terms of high performance and compactness. For example, the lenses described in Japanese Laid-Open Patent Applications H09-166748, 2000-047101, and 2000-105334 all have a configuration in which the third and fourth lenses from the object side are negative and positive lens elements joined together, which results in a tendency for the back focus of the lenses to be excessively long, which, in turn, prevents the lens from being compact. Also, the lens described in Japanese Laid-Open Patent Application H11-142730 has the same problem of compactness in terms of its total length being at least four times its focal length.
The present invention relates to a fixed focal length, wide-angle, high performance imaging lens that is compact and has a relatively simple configuration of lens elements that includes at least one plastic lens element and at least one lens element with an aspheric lens surface.